3rd Generation Partnership Project Long Term Evolution (3GPP LTE) adopts Orthogonal Frequency Division Multiple Access (OFDMA) as a downlink communication scheme.
In radio communication systems to which 3GPP LTE is applied, a base station (hereinafter, may be referred to as “eNB”) transmits a synchronization signal (i.e., Synchronization Channel: SCH) and a broadcast signal (i.e., Physical Broadcast Channel: PBCH) using a predetermined communication resource. Each terminal (hereinafter, may be referred to as “UE” (User Equipment)) finds the SCH, first and thereby ensures synchronization with the base station. Subsequently, the terminal reads BCH information to acquire a base station-specific parameter (e.g., frequency bandwidth) (e.g., see, Non-Patent Literature (hereinafter, abbreviated as “NPL”) 1, 2 and 3).
In addition, upon completion of the acquisition of the base station-specific parameter, each terminal makes a connection request to the base station to thereby establish a communication link with the base station. The base station transmits control information via a control channel such as Physical Downlink Control Channel (PDCCH) as appropriate to the terminal with which the communication link has been established. The terminal performs “blind-determination” on each of a plurality of pieces of control information included in the received PDCCH signal. More specifically, each of the pieces of control information includes a Cyclic Redundancy Check (CRC) part and the base station masks this CRC part using the terminal ID of the transmission target terminal. Accordingly, until the terminal demasks the CRC part of the piece of the received control information with its own terminal ID, the terminal cannot determine whether or not the piece of control information is intended for the terminal. In this blind-determination, if the result of demasking of the CRC part indicates that the CRC operation is OK, the piece of control information is determined as being intended for the terminal.
Moreover, in LTE, Hybrid Automatic Repeat Request (HARD) is applied to downlink data to terminals from a base station. More specifically, each terminal feeds back a response signal indicating the result of error detection on the downlink data to the base station. Each terminal performs a CRC on the downlink data and feeds back an Acknowledgment (ACK) when CRC=OK (no error) or Negative Acknowledgment (NACK) when CRC=Not OK (error) to the base station as a response signal. An uplink control channel such as Physical Uplink Control Channel (PUCCH) is used to send the response signal (i.e., ACK/NACK signal) as feedback.
The control information to be transmitted from the base station herein includes resource allocation information including information on a resource allocated to the terminal by the base station. As described above, PDCCH is used to transmit this control information. This PDCCH includes one or more L1/L2 control channels (L1/L2 CCH). Each L1/L2 CCH includes one or more Control Channel Elements (CCE). More specifically, a CCE is the basic unit used to map the control information to PDCCH. Moreover, when a single L1/L2 CCH includes a plurality of CCEs, contiguous CCEs are allocated to the L1/L2 CCH. The base station assigns the L1/L2 CCH to the resource allocation target terminal in accordance with the number of CCEs required for indicating the control information to the resource allocation target terminal. The base station maps the control information to a physical resource corresponding to the CCE of the L1/L2 CCH to transmit the control information.
In addition, CCEs are associated with PUCCH component resources (hereinafter, referred to as “PUCCH resource”) in one-to-one correspondence. Accordingly, a terminal that has received an L1/L2 CCH identifies the PUCCH resources corresponding to the CCEs forming the L1/L2 CCH and transmits an ACK/NACK signal to the base station using the identified PUCCH resources. However, when the L1/L2 CCH occupies contiguous CCEs, the terminal transmits an ACK/NACK signal to the base station using one PUCCH resource among the plurality of PUCCH resources respectively corresponding to the CCEs (e.g., PUCCH resource corresponding to a CCE having the smallest index).
As illustrated in FIG. 1, the transmission timing of an ACK/NACK signal on PUCCH from a terminal is in or after a K-th subframe from a subframe in which the received PDCCH signal and Physical Downlink Shared Chanel (PDSCH) to which data is assigned by the PDDCH signal are received (i.e., subframe n in FIG. 1) (e.g., K=4 in Frequency Division Duplex (FDD) (i.e., subframe n+K in FIG. 1).
A plurality of ACK/NACK signals transmitted from a plurality of terminals are spread using a Zero Auto-correlation (ZAC) sequence having the characteristics of zero auto-correlation in time-domain, a Walsh sequence and a discrete Fourier transform (DFT) sequence, and are code-multiplexed in PUCCH as illustrated in FIG. 2. In FIG. 2, W(0), W(1), W(2), W(3) represent a length-4 Walsh sequence and F(0), F(1), F(2) represent a length-3 DFT sequence.
As illustrated in FIG. 2, in the terminals, ACK/NACK signals are primary-spread over frequency components corresponding to 1 Single-Carrier Frequency Division Multiple Access (1SC-FDMA) symbol by a ZAC sequence (length-12) on frequency-domain first. In other words, the length-12 ZAC sequence is multiplied by an ACK/NACK signal component represented by a complex number. Subsequently, the primary-spread ACK/NACK signals and a ZAC sequence serving the reference signals are secondary-spread using a Walsh sequence (lengh-4: W(0) to W(3)) and a DFT sequence (length-3: F(0) to F(2)). More specifically, each component of the length-12 sequence signal (i.e., primary-spread ACK/NACK signals or a ZAC sequence serving as reference signals) is multiplied by each component of an orthogonal code sequence (i.e., Walsh sequence or DFT sequence). Moreover, the secondary-spread signals are transformed into a length-12 sequence signal in the time-domain by inverse discrete Fourier transform (IDFT) (or inverse fast Fourier transform (IFFT)). A cyclic prefix (CP) is added to each signal obtained by the IFFT, and a signal of one slot consisting of seven SC-FDMA symbols is thus formed.
PUCCH is mapped to both ends of a system band in the frequency domain. In PUCCH, a radio resource is allocated to each terminal in units of subframes. Each subframe consists of two slots, and for PUCCH, frequency hopping is applied between the first slot and last slot (inter-slot frequency hopping).
ACK/NACK signals from different terminals are spread using ZAC sequences corresponding different cyclic shift values (i.e., cyclic shift index) or orthogonal code sequences corresponding to different sequence numbers (i.e., orthogonal cover index (OC index)). An orthogonal code sequence is a combination of a Walsh sequence and a DFT sequence. In addition, the orthogonal code sequence is referred to as a block-wise spreading code in some cases. Thus, base stations can demultiplex the code-multiplexed ACK/NACK signals, using the conventional despreading and correlation processing (e.g., see, NPL 4). FIG. 3 illustrates PUCCH resources defined by sequence numbers of orthogonal code sequences (OC index: 0 to 2) and cyclic shift values (i.e., cyclic shift index: 0 to 11) of a ZAC sequence. When a length-4 Walsh sequence and a length-3 DFT sequence are used, a single subcarrier includes a maximum of 36 PUCCH resources (3*12=36). However, it is not always true that the 36 PUCCH resources are all made available. For example, FIG. 3 illustrates a case where 18 PUCCH resources (#0 to #17) are made available.
It is worth noting that, as an infrastructure to support the future information society, Machine-to-Machine (M2M) communication, which enables a service using inter-device autonomous communication without involving user judgment, has been considered as a promising technology in recent years. Smart grid is a specific application example of the M2M system. Smart grid is an infrastructure system that efficiently supplies a lifeline such as electricity or gas, performs M2M communication between a smart meter provided in each home or building and a central server, and autonomously and efficiently brings supply and demand for resources into balance. Other application examples of the M2M communication system include a monitoring system for goods management or remote medical care, or remote inventory or charge management of vending machines.
In the M2M communication system, use of a cellular system supporting a broad range of a communication area is particularly attracting attention. In 3GPP, studies on M2M in the cellular network have been carried out in LTE and LTE-Advanced standardization under the title of “Machine Type Communication (MTC).” In particular, “Coverage Enhancement,” which further extends the communication area, has been studied in order to support a case where an MTC communication device is installed at a location not usable in the current communication area, such as a smart meter in the basement of a building (e.g., see NPL 5).
In the MTC coverage enhancement, in particular, a technique called “repetition” that repeatedly transmits the same signal multiple times is considered as an important technique for extending the communication area. More specifically, repetition transmission is expected to be performed on channels such as PDCCH, PDSCH, and PUCCH.